This invention relates to a process for the production of ceramic and vitreous coatings, in particular glazes and engobes, comprising electrostatic application of a coating powder onto a ceramic substrate and firing of the coated substrate. The invention furthermore relates to a coating powder particularly suitable for the performance of the process and to the use thereof.
Ceramic and vitreous coatings, such as engobes and glazes, on ceramic, in particular unfired or partially fired, substrates are predominantly produced using aqueous slips. After application of the slip, the substrate coated therewith is fired, wherein the stovable material contained in the slip melts or sinters together to yield a ceramic or vitreous layer. Due to the disadvantages associated with the use of aqueous slips, such as effluent problems and elevated energy consumption, electrostatic powder coating of ceramic products is becoming increasingly significant.
Providing ceramic substrates, such as porcelain, earthenware and stoneware, but in particular unfired or only partially fired substrates, with an electrostatic coating still occasions various problems with regard to electrostatic application of the coating powder, inadequate adhesion of the powder to the substrate and, often, inadequate handling resistance. In addition to these problems, there are glazing defects which only occur on firing.
Various approaches have been used to solving the electrostatic and adhesion problems: thus, according to DE-PS 29 41 026, glaze powder may be adjusted to a specific surface resistance value suitable for electrostatic application of greater than 1xc2x71010 Ohmxc2x7m by coating with a polysiloxane. Engobes may also be applied electrostatically as powders after such hydrophobing treatment (EP-A 0 442 109). While, according to DE-A 42 39 541, the adhesion of an electrostatically applied glaze powder may indeed be improved by initially applying an aqueous coupling layer containing a polymer and a glass frit onto the substrate, the use of an aqueous system is regarded as disadvantageous.
WO 94/26679 discloses an improvement to the adhesion and handling resistance of a stovable coating powder, such as a glaze powder, which has been electrostatically applied to a substrate: in this case, the coating powder contains, apart from a glaze powder, a chemically or physically activatable coupling agent which combusts without leaving a residue on firing, such as polyolefins and dextrins, by means of which, after activation, the particles of the layer are fixed to each other and to the substrate. Preferred coating powders contain polysiloxane-coated glass frits mixed with to 15 wt. % of thermoplastic or 5 to 10 wt. % of dextrin. Usable glazes may be obtained on porcelain biscuit bodies only under specially optimised conditions, which, however, entail increased costs. If the conditions are only slightly modified, depending upon the substrate, sometimes considerable glaze defects and deficiencies occur before the required layer thickness is obtained.
WO 97/08115 discloses a remedy to the above-stated problems: the production process may be simplified without degrading glaze quality by using a glaze or engobe composition having a particular grain size distribution, namely a d50 value of 5 to 25 xcexcm, a d90 value of less than 35 xcexcm and a d10 value of greater than or equal to 2 xcexcm, in a coating powder additionally containing a coupling agent.
While the two above-mentioned processes do allow the production of defect-free glazes on fired or biscuit-fired porcelain bodies, when the same methods are used to produce glazes and engobes on unfired bodies, such as wall and floor tiles, the stoved coating exhibits considerable deficiencies in quality. In the case of biscuit-fired bodies too, the frequency of glaze defects is often excessively high. The defects often take the form of large, xe2x80x9cfrozen-inxe2x80x9d blisters and extensive areas without glaze (the occurrence of such defects is also known as xe2x80x9ccrawlingxe2x80x9d). These defects often occupy 10 to 30% of the stated area. The cause of this defect is suspected to involve the following interactions: the electrostatically applied layer is much looser and moreover exhibits much lower adhesion to the substrate than the layer obtained when a conventional slip is used. During firing of a green body provided with a glaze layer, gas-forming reactions occur in the green body, in the boundary layer between the body and the glaze layer and in the glaze layer, such reactions including, for example, the conversion of kaolin into metakaolin, which in particular proceeds at a temperature in the range from approx. 700 to 800xc2x0 C., and the elimination of CO2 from carbonates, such as dolomite, present in the body, which occurs at around 900xc2x0 C. If the layer is too thin and adheres inadequately, it is labile from when an initially present organic binder has combusted during the firing until the temperature at which the glaze melts, such that it may be destroyed by slight mechanical action, such as vibration and air currents in the kiln and by degassing processes in the body. One of these disruptions is manifested by lifting of an area of the electrostatically applied layer, so resulting in the formation of blisters and glaze-free areas once the blisters have burst. Clearly, the capillary forces which occur when a slip is applied using conventional methods firmly anchor the layer to the green body, such that the stated disruptions do not occur, whereas such anchoring is absent when the layer is applied electrostatically.
The object of the present invention is accordingly to overcome the stated problems in the production of ceramic and vitreous coatings, in particular glazes, wherein the coating powders are applied electrostatically. Once fired, the coatings may contain crystalline and/or amorphous fractions.
A process has been found for the production of a ceramic and/or vitreous coating, in particular a glaze, on a ceramic substrate, comprising electrostatic application of a coating powder and firing of the coated substrate, which process is characterized in that a coating powder CPAB is used which contains 1 to 50 wt. % of a layer-forming composition A having a softening onset in the range from 400 to 750xc2x0 C. and 99 to 50 wt. % of a layer-forming composition B having a softening onset in the range from above 750 to 1100xc2x0 C., or in that an underlayer is applied onto the ceramic substrate using a coating powder CPU containing at least 5 wt. % of the layer-forming composition A and an upper layer is applied thereon using a coating powder CPO containing at least 50 wt. % of the layer-forming composition B, wherein at least one of the two layers is applied electrostatically and the underlayer constitutes at least 1% of the total coating. The term xe2x80x9clayer-formingxe2x80x9d is taken to mean that, on firing, the composition is capable of forming a layer of a crystalline and/or amorphous, i.e. ceramic and/or vitreous, material.
The essence of the invention is the use of a layer-forming composition A, which has a softening onset T(EB)A below the softening onset T(EB)B of a layer-forming composition B, wherein composition B constitutes the majority of the ceramic coating to be produced. The layer-forming composition A preferably comprises a low-melting glass composition. The majority of the coating comprises compositions as are used for glazing, engobing and otherwise decorating ceramic substrates. According to the invention, the ceramic substrates to be coated in particular comprise those in which or at the boundary layer of which with the applied coating degassing processes occur during firing; such substrates include unfired or biscuit-fired earthenware and stoneware as well as unfired and biscuit-fired porcelain. Unfired wall and floor tiles and roofing tiles are particularly suitable substrates. Composition A is either a constituent in an effective quantity of the coating powder CPAB directly applied as a single layer or a constituent in an effective quantity of a coating powder CPU for a lower layer, over which an upper layer of a coating powder CPO of a different composition is applied.
The function of composition A is considered to be that it :effects good adhesion of the coating to the substrate during the stoving operation. Composition A is here selected such that its softening onset is preferably below the temperature at which degassing processes occur within the substrate, at the boundary layer and within the coating. Since composition A melts before the said degassing processes occur, in the case of the single-layer structure preferred according to the invention using the coating powder CPAB, both good adhesion to the substrate and good cohesion of the coating itself are effected during firing. Blistering, displacement of the entire layer, for example due to vibration in a sliding-bat kiln, or blowing off in a stationary kiln operated by firing are thus avoided. If the optimum composition and quantity to be used are selected, the area of crawling (=total area of defects) may be reduced to zero. Degassing from the substrate may proceed in part through the reverse side thereof and/or through the sintered coating.
In the alternative two-layer structure, the lower layer effects adhesion to the substrate. Even if the lower layer is much thinner than the upper layer arranged thereon, crawling is avoided during firing. The thickness of the lower layer is at least 1% and generally less than 50%, in particular 2 to 30%, of the total thickness of the two layers. Applying a low-melting glass flux as a thin lower layer has the advantage that the properties of the upper layers which are vital for service are much less affected.
After firing, the total thickness of the single or two-layer ceramic and/or vitreous coating is within the conventional limits known from the prior art, for example from glazing and engobing using aqueous slips, i.e. usually in the range from 50 to 1000 xcexcm, in particular from 100 to 500 xcexcm, and in the case of glazes particularly preferably in the range from 200 to 300 xcexcm.
While the chemical composition of the layer-forming composition A, which in both alternative embodiments is a constituent of the layer in contact with the ceramic substrate, does substantially determine the softening onset and the course of melting, determined as T(EB) and T(HK) (=hemisphere point) under a heating microscope, it is, however, of little significance to the final result of the single or two-layer coating. Composition A may assume the form of a glass frit or of a pulverulent mixture of substances, which begins to soften at T(EB) to form glass. Glass frits are preferred. When a mixture of substances is used, it conveniently assumes the form of a previously homogenised and spray-pelletised powder. It is known that low softening point glasses, which are also known as fluxes in decorative applications, are characterized by elevated contents of oxides from the range comprising PbO, Bi2O3, ZnO, B2O3 and alkali metal oxides. Where an unfritted composition A is used, it may consist of one or more substances from the range comprising alkali metal borates, alkali metal silicates, lead borosilicates, bismuth borosilicates and zinc borosilicates. 80 to 100 wt. % of composition A, relative to the glass-forming components, preferably assume the form of a glass frit.
Composition A is conveniently used in ultra-finely ground form. Effective action is then achieved at a low usage rate if the grain size range is substantially finer than that of the layer-forming composition B. The d50 value of composition A is preferably in the range from 1 to 5 xcexcm and the d90 value is below the d50 value of composition B.
The softening onset of the composition A to be used is usually in the range from 400 to 750xc2x0 C., in particular in the range from 450 to 600xc2x0 C. A softening onset of around 500xc2x0 C. is particularly preferred, because it is then ensured that, on firing, once an organic coupling agent which is additionally present in the coating powder and is initially responsible for good adhesion and handling resistance of the layer has combusted, the layer-forming composition A exerts its action.
Layer-forming composition B comprises such a composition which begins to soften at around/above 750xc2x0 C., preferably at 800 to 1050xc2x0 C. and in particular in the range from 900 to 1000xc2x0 C. Composition B should not begin to soften until composition A has melted to such an extent that the required adhesion to the substrate and within the coating containing composition B has come into effect. The chemical composition of composition B corresponds to that as is conventional for coatings of this generic type, such as in particular glazes and engobes. The person skilled in the art is familiar with such compositions; reference is made by way of example to Ullmann""s Encyclopedia of Industrial Chemistry, 5th edition, 1986, pp. 31-33. Glazes which are used according to the invention preferably comprise systems, the majority of which assumes the form of a glass frit, but a smaller proportion of which, namely up to 30 wt. %, preferably up to 10 wt. %, may be present in the form of further components, such as clay minerals, such as kaolin, and/or nepheline syenite. Such systems are in particular suitable for so-called rapid single firing glazes, as are used for glazing wall tiles, which are fired at approximately 1100xc2x0 C. (xc2x150xc2x0 C.). Glazes for floor tiles are more strongly formulated to a T(EB) of around/above 900xc2x0 C. and a firing temperature of around 1200xc2x0 C. Such glazes conventionally contain a smaller proportion of glass frits; example compositions substantially contain (wt. %) 30 to 50% glass frit, 5 to 15% wollastonite (Ca silicate), 5 to 15% alumina (Al2O3), 0 to 15% zirconium silicate, 5 to 15% kaolin plus colouring pigments if required. Glazes based on the above-stated mixtures of substances are conveniently used in the form of sprayed pellets.
The substantial constituents of engobes are glass frits, finely divided ceramic raw materials, ground minerals, glass and porcelain flour, together with opacifiers and/or pigments. In this case too, it is convenient to use spray-dried pellets because segregation is avoided and uniform melting behaviour is achieved.
Composition B preferably consists of 30 to 100 wt. % of one or more glass frits.
According to a preferred embodiment, both the coating powder CPAB to be sprayed for the single layer coating and the powders CPO and optionally also CP, to be used for the two-layer coating, have a grain size range as disclosed in WO 97/08115 of d50 5 to 25 xcexcm, d10xe2x89xa72 xcexcm and d90 less than 35 xcexcm.
The powder of both composition A and B or of the mixture containing A and B may, where required for electrostatic reasons, have a known hydrophobing outer coat, for example a polysiloxane outer coat.
The coating powder CPAB to be used for the single layer coating contains 1 to 50 wt. % of composition A and 50 to 99 wt. % of a composition B. Preferably, however, CPAB contains 2.5 to 25 wt. % of A and 97.5 to 75 wt. % of B, in each case relative to all the ceramic and/or vitreous layer-forming components. Coating powder CPAB preferably contains a total of 75 to 95 wt. %, in particular 90 to 95 wt. % of ceramic and/or vitreous layer-forming components; where CPAB contains relatively large quantities of colouring pigments, the total of A and B may be still lower. Compositions A and B may each contain two or more glass frits, the T(EB) values of which are within the claimed range. An excessively large proportion of A in CPAB with an excessively low T(EB)A should be avoided because an undulating surface and/or pinholing of the glaze may occur. The person skilled in the art will perform initial investigatory testing to determine the blend of composition B with composition A with regard to T(EB)A and the quantity of A relative to B which is most suitable for obtaining a defect-free glaze or engobe or decoration.
For the purposes of two-layer coating, the lower layer may have a composition similar to that of CPAB but with at most 5 wt. % of composition A and up to 95 wt. % of composition B. Alternatively, the lower layer may contain exclusively a composition according to A. In this case, the layer thickness of the lower layer should, if at all possible, constitute no more than 10% of the total thickness of both layers. The upper layer contains a layer-forming composition B as the main component (xe2x89xa750%) of the coating powder CPO. Coating powder CPO may, however, also contain a composition A, specifically in a smaller quantity than it is present in CPU.
It is known that electrostatically applicable coating powders, preferably glazes, may, apart from ceramic and/or vitreous layer-forming components, also contain one or more chemically or physically, in particular thermally, activated coupling agents. This is also the case for the process according to the invention and for the coating powders to be used for this purpose, such that these preferably contain an effective quantity of such coupling agents. Such materials which may be used are, for example, thermoplastics and chemically curing reaction resins and moisture-activatable substances. Explicit reference is hereby made to WO 94/26679 and WO 97/08115 with regard to the selection of materials, the quantity to be used and the function of the coupling agents. Suitable coupling agents which are activatable by thermal treatment are thermoplastic homo- and copolymers having a softening point in the range between 60 and 250xc2x0 C., preferably between 80 and 200xc2x0 C. and in particular between 80 and 150xc2x0 C. The thermoplastic coupling agents preferably comprise polyolefins, such as paraffin wax and low density polyethylene (LDPE), together with acrylate and methacrylate polymers and copolymers, polyvinyl compounds, such as polystyrene, polyvinyl acetate, ethylene/vinyl acetate copolymers, styrene/acrylate copolymers; polyesters and copolyesters as well as polyamides and copolyamides are also usable.
According to another embodiment, hydrophilic and hydrophobic polymers are combined together as thermally activatable coupling agents in such a manner that the glaze powder optionally still containing residual moisture and/or the substrate to be glazed are reliably wetted. Hydrophilic polymers exhibit structural elements which are capable of forming hydrogen bridges, for example hydroxyl groups, carboxyl groups and/or ether bridges.
It is advantageous if the diameter of the particles of the coupling agent(s) used in the coating powder is within the particle size range of the stovable compositions. The average particle diameter d50 of the coupling agent is particularly preferably below the d50 value of the stovable compositions. A narrow grain size range and moreover a spherical grain habit of the coupling agent are particularly preferred.
Chemical activation comprises, for example, polymerisation, such as crosslinking in the presence of a polyfunctional acrylate or methacrylate, or a polyaddition or polycondensation reaction of two-component systems. Physical activation preferably comprises partial melting of the coupling agent with subsequent cooling and solidification. This type of activation may be effected by heating the substrate to be coated before, during or after electrostatic coating and may proceed using conventional ovens or irradiation with infra-red light sources.
It has been found that a coating powder containing one or more stovable compositions and one or more coupling agents from the range comprising polyolefins, in particular polyethylene, should conveniently be applied electrostatically within approximately one day of the production thereof in a high intensity mixing or grinding unit, in order to avoid possible ageing-related disadvantages.
It has been found that, with regard to obtaining defect-free stoved coatings, it is particularly advantageous to incorporate the coupling agent into the layer-forming powder by means of an intensive mixing or grinding process. The temperature occurring during incorporation must be below the activation temperature of a coupling agent activatable by melting or a chemical reaction. Particularly advantageously usable intensive mixing and grinding units contain a high speed rotating beater, which is operated at a rotational speed in the range from 2000 to 20000 rpm, in particular at approximately 5000 to 15000 rpm. The stated measure of incorporating one or more coupling agents into a coating powder containing one or more stovable layer-forming compositions by means of a, high intensity mixer or high intensity mill is advantageous for any coating powders which are to be applied electrostatically, i.e. also those which are applied onto glass or fired ceramic substrates or metal. A high intensity mill from the range comprising beater and jet mills is preferably used for this purpose. The rotational speed of the beater mills, such as a pin mill or pin beater mill, should be as high as possible during mixing. The rotational speed will usually be in the range from 2000 to 20000 rpm (revolutions per minute), particularly 5000 to 15000 rpm.
According to a preferred embodiment, coupling agents having a narrow grain size range are used for the production of coating powders containing coupling agents, for example polyethylene wax having a grain size range of substantially 1 to 20 xcexcm, in particular approximately 5 to approximately 10 xcexcm, for 90% of the powder.
The coating powders CPAB and CPO and/or CPU may additionally contain pigments which are stable under the firing conditions. Pigment content will not generally exceed 20 wt. %. Coloured coatings may also be obtained by using coloured frits.
The coating powders CPAB, CPO and CPU may additionally contain auxiliaries in a quantity of generally up to 5 wt. %, but usually only of up to 2 wt. %, for the purposes of trouble-free processing. Examples are fluidising auxiliaries. Fluidising auxiliaries in particular comprise pyrogenically produced oxides, which may in turn be hydrophobed Suitable fluidising agents are, for example, silica, titanium dioxide and aluminum oxide and ZrO2. Prior art coating powders often contain such fluidising auxiliaries in a quantity of between 0.5 and 3 wt. %; preferred coating powders according to the invention contain 0 to 0.3 wt. %, in particular 0 to 0.2 wt. %, of fluidising agent, for example pyrogenic SiO2 (AEROSIL(copyright) from Degussa AG), relative to stovable material.
Further auxiliaries optionally present in the coating powders are those with which the electrical properties of the powders may be modified in such a manner that their specific electrical resistance permits trouble-free electrostatic spraying. Examples of such auxiliaries are hydrophobing agents. The specific electrical resistance of the coating powders should generally be within the range from approximately 109 to approximately 1014 Ohmxc2x7m.
Application of the coating in one or two layers by electrostatic spraying proceeds in a manner known per se using a high voltage gun operating in accordance with the corona or super-corona principle. Voltage is conventionally to 100 kV, in particular 40 to 80 kV, the current 40 to 80 xcexcA.
According to a preferred embodiment of the process, the substrate to be coated is preheated before spraying, conveniently to 100 to 250xc2x0 C., because, as disclosed in WO 97/08115, this improves the adhesion of the powder to the substrate and so facilitates handling of the unfired coated substrate. Pretreating the substrate with a salt according to WO 94/26679 is also possible.
The coated substrate is fired in a conventional manner in known kilns. The firing temperature is determined by the composition of the unfired or biscuit-fired substrate and of the coating, but is conventionally in the range above approximately 900xc2x0 C. up to approximately 1450xc2x0 C., usually 1000 to 1300xc2x0 C.
The present invention also provides an electrostatically applicable coating powder CPAB comprising a composition which forms a layer on ceramic firing, which powder is characterized in that it contains from 1 to 50 wt. % of a layer-forming composition A having a softening onset in the range from 400 to 750xc2x0 C. and 99 to 50 wt. % of a layer-forming composition B having a softening onset in the range from above 750 to 1100xc2x0 C. The coating powder CPAB preferably contains 2.5 to 25 wt. % of a layer-forming composition A and 75 to 97.5 wt. % of a layer-forming composition B, in each case relative to layer-forming components, i.e. the sum of A and B. A particularly preferred coating powder substantially consists of 75 to 95 wt. % of a composition B, 3 to 10 wt. % of a composition A, 2 to 10 wt. % of a thermally or chemically activatable organic polymer, 0 to 2 wt. % of fluidising agents and auxiliaries to establish the specific surface resistance, such as a carboxylic acid salt. The layer-forming compositions A and B in the coating powder CPAB preferably exhibit a d10 value of 5 to 25 xcexcm, a d90 value of less than 35 xcexcm and a d10 value of greater than or equal to 2 xcexcm. The d10, d50 and d90 values state the grain diameter at which 10%, 50% and 90% of the grains pass through (determined to DIN 66141, for example using the CILAS HR 850-B granulometer).
By applying the process according to the invention in the alternative embodiments and using the coating powder according to the invention for single layer coating, it is possible to glaze, engobe or decorate unfired and biscuit-fired substrates without the hitherto obtained extensive areas of glaze defects occurring on firing. The coating powder CPAB may be produced in a simple manner by intensive mixing of the constituents. Selection of the constituents occasions no problems because they comprise raw materials or intermediates, such as in particular glass frits, conventional in the ceramics industry, as well as known auxiliaries. The coating powders according to the invention are particularly suitable for the production of glazed tiles using the rapid single firing process, wherein unfired tiles are coated electrostatically and then fired (Monoporora process).